BACKGROUND
BACKGROUND
The relation between the pilot input on the stick and the aircraft response is called the CONTROL LAW and as such determines the handling
characteristics of the A/C.
The Fly By Wire system comprises 3 sets of control laws depending upon the integrity and redundancy status of the computers, peripherals and hydraulic systems. These three control laws are called NORMAL, ALTERNATE and DIRECT LAW.
This following information provides an overview of the flight control laws on the FBW aircraft and the protections provided to the pilot.
SIDE STICK AND PRIORITY P/B NORMAL LAW
ALTERNATE LAW DIRECT LAW
MECHANICAL BACK-UP ABNORMAL ATTITUDE LAW ALPHA FLOOR
LOW ENERGY WARNING ALPHA LOCK
ALPHA LOCK
AUTOMATIC RETRACTION SYSTEM REACTION TO ENGINE FAILURE SIDE STICK AND PRIORITY P/B
When a pilot moves the side stick, he sends an order (an electrical signal) to the FBW
computer. Thus if the PNF acts on the stick as well, both signals or orders are added.
Thus the PF and PNF shall not act on the stick simultaneously.
If the PNF needs to take over, he must press the priority P/B and announce
"I have control".
In case of a pilot who collapses on the stick, or in case of a mechanical side stick failure leading to a jammed stick the "failed" stick order is added to the
"non failed" side stick order.n such a case, the pilot will press the priority P/B during at least 30 sec. in order to deactivate the "failed" side stick.
In case of a SIDE STICK FAULT ECAM warning due to an electrical failure, the affected side stick order sent to the computer is forced to zero; in other words the affected side stick is deactivated.
When a side stick is deactivated by the opposite side stick priority P/B, it can be reactivated by pressing its own priority P/B.
NORMAL LAW
Normal Law is the normal case and allows for all “single” failures. The
handling characteristics of Normal Law (within the normal flight envelope and, regardless of IAS, Altitude, Gross Weight and C of G), are such that the aircraft is stable, with the same consistent response. The efforts on the side stick are balanced in pitch and roll.
Don’t fight with the stick - if you feel you are over controlling, release it.
There are three modes of normal law; ground, flight and flare mode.
a. Ground Mode
Direct control of elevator, spoilers, ailerons and rudder. This is progressively blended out when airborne so that flight mode becomes effective.
b. Flight Mode
Side-stick movement in the pitch axis commands a change in ‘g’. Zero displacement is a positive command for 1g flight. 1g flight means no change in flight path. So once the correct flight path has been established, in the short term it will be maintained, despite any changes to thrust or speed. Hence there is no need to trim. Side-stick movement in the roll axis commands a given rate of roll. Zero displacement is a positive command for zero roll rate flight. Once the required bank angle has been established, release the side-stick to neutral and it will be maintained.
Make a small input then gently release to neutral and leave the stick alone unless a further adjustment is required. All turns may require some side-stick and power adjustments.
c. Flare Mode
A change in pitch control below 50 ft, requiring a gentle pull on the sidestick during the flare in order to maintain a progressive flare.
d. Protections: Full flight envelope protection is provided in normal law using the following individual protections.
These protections have been designed to assist pilots in emergency
situations, where under stress conditions only an instinctive and rapid reaction will save the situation. The protections make this reaction possible.
ALTERNATE LAW
In some cases of double failure, the integrity and redundancy of the
computers and peripherals are not sufficient to maintain Normal law with its protections.
The degradation is progressive depending upon the availability of remaining peripherals or computers. Depending on the particular failure that causes the degrading of the flight control laws, the ECAM will indicate whether Alternate Law with or without protections is active. The indications will be ALTN LAW or
ALTN LAW: PROT LOST. The main differences between these two laws and normal law are detailed below.
In alternate law pitch control is similar to normal law with some changes in the protections available. Autotrim is still available. Roll control is the same as direct law with the sidestick demanding aileron deflection, rather than roll rate.
Load factor limitation same as normal law Pitch attitude protection not provided
High angle of attack protection changed to low speed stability (PFD display also changes, VSW is shown and stall warning is provided)
High speed protection changed to alternate high speed stability In some failure cases alternate law without protection is available. All protections except load factor limitation are lost.
Protection in Alternate Law at the limits of the Flight Envelope:
At high speed, natural aircraft static stability is restored with an OVER SPEED WARNING (as the a utotrim stops on reaching the red line the aircraft is now flying faster than it is trimmed for so will naturally pitch up).
At the other end of the envelope at low speed, the auto pitch trim stops at Va prot (below VLS) and natural longitudinal static stability is restored, with STALL WARNING at 1.03 VS1g. (as the autotrim stops and the aircraft is flying slower than it is trimmed for there is a natural pitch down movement. If this is resisted the aircraft slows further and eventually stalls).
Summary of ALTN law:
Within the Normal Flight Envelope, the handling characteristics are the same in pitch as with the normal law and outside the Normal Flight Envelope, the pilot must take proper preventive actions to avoid loss of control, or high speed excursions as he would do it on any non protected A/C.
Note that, in ALTN law VMO is reduced to 320 kt and that A.FLOOR is inhibited.
DIRECT LAW
On the A320 in every case where the Landing Gear is extended when in Alternate Law (and in certain other cases) the control law becomes Direct. In Direct Law the elevator deflection is proportional to stick deflection (the maximum deflection is a function of CONF and CG) and aileron and spoiler deflections are proportional to stick deflection but vary with the A/C CONF and pitch trim is no longer automatic but is controlled manually by pilot input on the trim wheel. The ECAM will indicate to the crew when direct law is the active flight control law and USE MAN PITCH TRIM will be displayed on the PFD as a reminder. No protections are available, but overspeed or stall warnings are still operational.
INDICATIONS
The degradation of control laws is indicated on the ECAM as well as on PFD.
On the PFD the availability of protections, in normal law, is shown by specific symbols (= i n green), and by the specific display of the low speed information on the speed scale.
When the protections are lost, amber crosses are displayed instead of the green protection symbols.
When automatic pitch trim is no longer available (in Direct Law) this is
indicated by the message USE MAN PITCH TRIM in amber on the PFD. (The message USE MAN PITCH TRIM indicates that Direct Law is active even if it is not possible to move the stabiliser).
Just by watching his main instrument the PFD, the pilot is immediately aware of his control law status and operational consequences
MECHANICAL BACK-UP
When the A320 was certified it was necessary to demonstrate control of the aircraft with a total loss of the FBW system. The most unlikely way for this to happen is for a loss of five flight control computers. In order of probability it is more likely (statistically 1 in 1000 billion) for a different series of failures to occur which leads to a similar situation (loss of Blue hydraulic plus SEC2 plus ELAC2). This second scenario is what we demonstrate to the trainees.
Although roll control is still available from the right hand aileron the aim of the exercise is to control the flight path using solely the pitch trim and the
rudder. MAN PITCH TRIM ONLY will be displayed in red on the PFD. There is no turn coordination and no protections are operative.
The autothrust, if engaged, will give to large pitch changes and control is considerably easier with manual thrust.
The aim of the demonstration is not to fly the aircraft accurately, but to keep the aircraft in a safe stabilised attitude, allowing the lost systems to be restored by computer reset.
ABNORMAL ATTITUDE LAW
If any of the following limits are exceeded Pitch 50° up, 30° down, Bank 125°, AOA +30°, -10° and Speed <440, >60, Mach <0,96, >0,1, (due to atmospheric disturbance for example) the Abnormal Attitude Law is invoked where pitch is ALTN with load factor protection (without autotrim) and lateral Direct Law with yaw alternate.
The FBW architecture and control laws explain why upset recovery manoeuvres need not be trained on Airbus protected A/C.
ALPHA FLOOR
Alpha floor is an autothrust mode, however it is also a part of the flight envelope protection.
At high angles of attack TOGA thrust is commanded by the autothrust system.
Alpha floor is available from lift off until 100 ft RA on approach. It provides protection against stall and windshear and has priority over all other
protections. Alpha floor is only available in normal law. It is inhibited is some cases.
LOW ENERGY WARNING
In Normal Law, a warning is included to alert the pilot to a low energy situation. It is not a protection, and occurs before alpha floor operates. This warning is only available below 2000 ft radio altitude and in CONF 2, 3 or FULL.
ALPHA LOCK
This protection prevents the retraction of flap from CONF I to zero, if speed is too low or AOA is too high.
AUTOMATIC RETRACTION SYSTEM
When accelerating through 210 kts with CONF I + F selected, the flaps will automatically retract, before VFE of 215 kt. The Flaps will not automatically re-extend if speed drops below 210 kt.
REACTION TO ENGINE FAILURE
The most efficient flying technique with regard to performance with an engine failure at take off is to fly a constant heading with roll surfaces retracted. This technique dictates the amount of rudder required, and the residual side slip which will result.
Consequently, in case of engine failure at take off smoothly control the pitch (lower) to keep the desired speed (as per SRS), and centre the Beta target with the rudder pedals. What your trainees need to understand is that if a conventional balance indicator were installed in our FBW aircraft it would not be centred if the Beta target is centred. This is evident during an approach when the single engine thrust increases above 80% N1 and the yellow slip
indicator changes into the blue Beta target but displaced from the slip index.
This is because a centred Beta target ensures all flight control surfaces are retracted and thus there will be a residual side slip. If the side slip index (when visible) is centred there will remain residual deflection of some flight control surfaces.